How to render PBR lens flares?
While Cycles is pretty physically accurate, this is not possible, or at least not practical without workarounds, as far as I know.

Lens flares are caused by tiny imperfections in the surface of the lenses or photographic equipment, which digital 3D models don't typically have.

You can introduce them by means of bump maps, displacement or material roughness, but it will probably lead to very noisy images and require an unpractical amount of samples.

It also probably has a component of light dispersion, and since Cycles is not a spectral render it will probably require further workarounds, making the physical accuracy questionable.

You can get decent approximations using the traditional methods involving compositor nodes for glare and light aberration.

Also see Can blender be used for simple optical modeling?